Prior art hydraulic circuit systems comprise, as disclosed in JP, B, 2-16416, first and second hydraulic pumps, a plurality of hydraulic actuators driven by hydraulic fluid delivered from the first and second hydraulic pumps, a first group of valves connected to a delivery line of the first hydraulic pump for controlling flow rates of hydraulic fluid supplied to the associated hydraulic actuators and a second group of valves connected to an delivery line of the second hydraulic pump for controlling flow rates of hydraulic fluid supplied to the associated hydraulic actuators.
The plurality of hydraulic actuators contains first and second traveling motors for driving right and left crawler belts of, for example, a hydraulic excavator and a plurality of working actuators except the first and second traveling motors including a swing motor of, for example, the hydraulic excavator for driving a swing, an arm cylinder for driving an arm, a boom cylinder for driving a boom and a bucket cylinder for driving a bucket.
The first group of valves includes a first traveling directional control valve for controlling a flow rate of hydraulic fluid supplied to the first traveling motor, and a plurality of first directional control valves for controlling flow rates of hydraulic fluid supplied to part of the plurality of working actuators including, for example, a swing directional control valve, a first arm directional control valve, and a first boom directional control valve, these first directional control valves being connected in tandem to the first traveling directional control valve so as to supply hydraulic fluid from the first hydraulic pump to the associated working actuators with priority over the first traveling motor. The second group of valves includes a plurality of second directional control valves for controlling flow rates of hydraulic fluid supplied to part of the plurality of working actuators including, for example, a second boom directional control valve, a bucket directional control valve and a second arm directional control valve, and a second traveling directional control valve for controlling a flow rate of hydraulic fluid supplied to the second traveling motor, the second traveling directional control valve being connected in tandem to the second directional control valves so as to supply hydraulic fluid from the second hydraulic pump to the second traveling motor with priority over the associated working actuators.
The hydraulic circuit system further contains a circuit for communication of a hydraulic fluid supply circuit of the second traveling directional control valve with a hydraulic fluid supply circuit of the first traveling directional control valve when at least one of a plurality of working actuators except the first and second traveling motors is operated. This communication circuit includes a branch line connecting the delivery line of the second hydraulic pump with an inlet port of the first traveling directional control valve, an on-off valve provided in the branch line for opening and closing thereof, and a check valve provided in the downstream of the on-off valve for preventing a reverse flow of hydraulic fluid. The on-off valve is maintained at its closed position when the first and second directional control valves associate with the working actuators are not activated, and switched to its open position when the first and second directional control valves are activated.
This prior art is intended mainly to improve performance of a combined operation of traveling while operating the swing, boom and arm, simultaneously.
For example, when performing a traveling operation alone, since the on-off valve is maintained at its closed position, all the hydraulic fluid from the first hydraulic pump is supplied to the first traveling motor through the first traveling directional control valve, and all the hydraulic fluid from the second hydraulic pump is supplied to the second traveling motor through the second traveling directional control valve. Consequently, the right and left crawler belts are driven to perform traveling.
If any one of the first directional control valves included in the first group of valves, for example, is operated in such a traveling condition, hydraulic fluid from the first hydraulic pump is preferentially supplied to the first directional control valve and since the on-off valve is switched to its open position, hydraulic fluid from the second hydraulic pump is supplied to the first and second traveling directional control valves. That is, the first and second traveling motors are supplied with hydraulic fluid from only the second hydraulic pump, thereby making it possible to implement the combined operation of traveling and performing other works.